1. Field of the Invention
The invention relates to an image pickup lens preferable as an optical system of a solid state pickup element such as a CCD type image sensor or a CMOS type image sensor, an image pickup unit having the image pickup lens, and a portable terminal having the image pickup unit.
2. Description of Related Art
In recent years, with the heightened performance and the miniaturization of an image pickup device using a solid-state image pickup element such as a CCD (charge coupled device) type image sensor or a CMOS (complementary metal oxide semiconductor) type image sensor, a portable terminal and a personal computer respectively having the image pickup device have spread to users.
Also, with the miniaturization and the dense arrangement of pixels of the element due to the increase of functions in the portable terminal and the personal computer, the further miniaturization of an image pickup lens mounted on the image pickup device is strongly required for the miniaturization of the image pickup device.
In recent years, because a triplet lens structure can have higher performance as compared with a single lens structure and a doublet lens structure, the triplet lens structure composed of a first lens having the positive refractive power, a second lens having the negative refractive power and a third lens having the positive refractive power arranged in that order from an object side has been generally used as an image pickup lens of a small-sized image pickup device. This triplet type image pickup lens is disclosed in Published Unexamined Japanese Patent Application (Tokukai) No. 2001-75006.
However, in the image pickup lens disclosed in the Application No. 2001-75006, though various types aberration are preferably corrected while maintaining the wide angle of view, the image pickup lens is not appropriate to the shortening of a total lens length (that is, a distance from an aperture stop to a focal point on an image side) along an optical axis.
In an order to solve the above problem, an object of the present invention is to provide an image pickup lens which is composed of a plurality of lenses and is miniaturized, an image pickup unit and a portable terminal.
In an order to accomplish the above-mentioned object, in accordance with the first aspect of the present invention, an image pickup lens comprising four lenses arranged in an order of a first lens, a second lens, a third lens and a fourth lens from an object side,
wherein the first lens has positive refractive power and has a convex surface facing toward the object side, the second lens has the positive refractive power, the third lens has negative refractive power and has a concave surface facing toward the object side to be formed in a meniscus shape, and the fourth lens has the positive or negative refractive power and has a convex surface facing toward the object side to be formed in the meniscus shape.
In the above configuration, four lenses are arranged in an order of the first, second, third and fourth lenses. In use, the first lens is placed on the object side, and the fourth lens is placed on an image side. In this use condition, the positive lens group and the negative lens respectively having the comparatively strong refractive power are arranged in that order from the object side, and the convex surface of the first lens faces toward the object side. Therefore, a total lens length is shortened. Here, the total lens length denotes a distance on an optical axis from the object side surface of the first lens to an image side focal point of the whole image pickup lens. However, in the image pickup lens having an aperture stop arranged nearest to the object side, the total lens length denotes a distance on an optical axis from the aperture stop to the image side focal point of the whole image pickup lens. This definition of the total lens length is effective in this specification including claims.
Also, the concave surface of the third lens formed in the meniscus shape faces toward the object side, and the convex surface of the fourth lens formed in the meniscus shape faces toward the object side. Therefore, an air lens having the positive refractive power is formed between the third and fourth lenses. Accordingly, the total lens length can be further shortened, and a telecentric characteristic can be maintained in the periphery of an image screen.
Here, the fourth lens has the shape and orientation of the convex surface described above and can have the positive or negative refractive power. Hereinafter, xe2x80x9cpositive or negative refractive powerxe2x80x9d denotes that any of the positive refractive power and the negative refractive power is allowed.
Preferably, following conditional formulas (1), (2) and (3) are satisfied:
L/2Y less than 1.60xe2x80x83xe2x80x83(1)
0.40 less than f12/f less than 0.70xe2x80x83xe2x80x83(2)
25 less than {(xcexd1+xcexd2)/2}xe2x88x92xcexd3xe2x80x83xe2x80x83(3),
where L denotes a distance on an optical axis from the object side surface of the first lens to an image side focal point of the whole image pickup lens, 2Y denotes the length of a diagonal line on an effective image screen, f12 denotes a combined focal length of the first lens and the second lens, f denotes a focal length of the whole image pickup lens, xcexd1 denotes an Abbe number of the first lens, xcexd2 denotes an Abbe number of the second lens, and xcexd3 denotes an Abbe number of the third lens.
The conditional formulas (1) to (3) indicate conditions for obtaining a miniaturized image pickup lens in which aberration is preferably corrected. Here, L of the conditional formula (1) denotes a distance on an optical axis from the object side surface of the first lens to the image side focal point of the whole image pickup lens. The xe2x80x9cimage side focal pointxe2x80x9d denotes an image point obtained when a parallel ray parallel to the optical axis is incident on the image pickup lens. Also, when an optical member of a plane parallel plate shape such as a low pass filter is arranged in a space between the surface of the image pickup lens nearest to the image side and the image side focal point, the distance L is converted into a distance L in air, and the distance L in air satisfies the formula (1).
The conditional formula (1) indicates a condition for achieving the shortening of the total lens length. When L/2Y is lower than the upper limit of the conditional formula (1), the total lens length is directed to be shortened.
Also, as to the conditional formula (2) for appropriately setting the combined focal length of the first lens and the second lens, when f12/f is set to be higher than the lower limit, the combined positive refractive power of the first lens and the second lens is not excessively enlarged, the generation of both the high-order spherical aberration and the coma can be suppressed. Also, when f12/f is set to be lower than the upper limit, the combined positive refractive power of the first lens and the second lens is appropriately obtained, and the total lens length can be shortened.
Also, as to the conditional formula (3) for the correction of the chromatic aberration in the first and second lenses having the positive refractive power and the third lens having the negative refractive power, when {(xcexd1+xcexd2)/2}xe2x88x92xcexd3 is set to be higher than the lower limit, the axial chromatic aberration and the lateral chromatic aberration can be corrected.
Preferably, an aperture stop is arranged nearest to the object side. Because of the configuration of the aperture stop arranged nearest to the object side, the position of an exit pupil can be set away from an image surface. Therefore, a principal ray of a light flux emerging from the final surface of the lenses is incident on a solid-state image pickup element at an angle near a right angle. That is, the image side telecentric characteristic required for the image pickup lens of the solid-state image pickup element can be preferably obtained, and a shading phenomenon in the periphery of an image screen can be reduced.
Preferably, the image pickup lens satisfies following conditional formulas (4), (5) and (6):
Lxe2x80x2/2Y less than 1.60xe2x80x83xe2x80x83(4)
0.40 less than f12/f less than 0.70xe2x80x83xe2x80x83(5)
25 less than {(xcexd1+xcexd2)/2}xe2x88x92xcexd3xe2x80x83xe2x80x83(6),
where Lxe2x80x2 denotes a distance on an optical axis from the aperture stop to an image side focal point of the whole image pickup lens, 2Y denotes the length of a diagonal line on an effective image screen, f12 denotes a combined focal length of the first lens and the second lens, f denotes a focal length of the whole image pickup lens, xcexd1 denotes an Abbe number of the first lens, xcexd2 denotes an Abbe number of the second lens, and xcexd3 denotes an Abbe number of the third lens.
The conditional formulas (4) to (6) indicate conditions for obtaining a miniaturized image pickup lens in which aberration is preferably corrected. Here, Lxe2x80x2 of the conditional formula (4) denotes a distance on an optical axis from the aperture stop to the image side focal point of the whole image pickup lens. The xe2x80x9cimage side focal pointxe2x80x9d denotes an image point obtained when a parallel ray parallel to the optical axis is incident on the image pickup lens. Also, when an optical member of a plane parallel plate shape such as a low pass filter is arranged in a space between the surface of the image pickup lens nearest to the image side and the image side focal point, the distance Lxe2x80x2 is converted into a distance Lxe2x80x2 in air, and the distance Lxe2x80x2 in air satisfies the formula (4).
The conditional formula (4) indicates a condition for achieving the shortening of the total lens length. When Lxe2x80x2/2Y is lower than the upper limit of the conditional formula (4), the total lens length is directed to be shortened.
Also, as to the conditional formula (5) for appropriately setting the combined focal length of the first lens and the second lens, when f12/f is set to be higher than the lower limit, the combined positive refractive power of the first lens and the second lens is not excessively enlarged, the generation of both the high-order spherical aberration and the coma can be suppressed. Also, when f12/f is set to be lower than the upper limit, the combined positive refractive power of the first lens and the second lens is appropriately obtained, and the total lens length can be shortened.
Also, as to the conditional formula (6) for the correction of the chromatic aberration in the first and second lenses having the positive refractive power and the third lens having the negative refractive power, when {(xcexd1+xcexd2)/2}xe2x88x92xcexd3 is set to be higher than the lower limit, the axial chromatic aberration and the lateral chromatic aberration can be corrected.
When an aperture stop is arranged nearest to the object side, the total lens length can be set to a distance from the aperture stop to the image side focal point of the whole image pickup lens, and the calculation based on the total lens length is preferred.
Preferably, following conditional formulas (7) and (8) are satisfied:
xe2x88x920.40 less than R5/((N3xe2x88x921)xc2x7f) less than xe2x88x920.20xe2x80x83xe2x80x83(7)
0.30 less than fa/f21 0.50xe2x80x83xe2x80x83(8),
where f denotes a focal length of the whole image pickup lens, R5 denotes a curvature radius of the object side surface of the third lens facing, N3 denotes a refractive index of the third lens at a d-line, and fa denotes a focal length of an air lens formed by an image side surface of the third lens and the object side surface of the fourth lens.
When the negative refractive power of the object side surface of the third lens is appropriately set, the conditional formula (7) indicates a condition for easily correcting a curvature of the image surface and flattening the image surface. Here, the focal length of the object side surface of the third lens is expressed by R5/(N3xe2x88x921) by using the curvature radius R5 and the refractive index N3 of the third lens. The conditional formula (7) indicates a ratio of the focal length of the object side surface of the third lens to the focal length of the whole image pickup lens.
When R5/((N3xe2x88x921)xc2x7f) is lower than the higher limit of the conditional formula (7), the negative refractive power of the object side surface of the third lens is not excessively enlarged, and the generation of excessive spherical aberration and the generation of coma flare of an abaxial light flux can be suppressed. When R5/((N3xe2x88x921)xc2x7f) is higher than the lower limit, the negative refractive power of the object side surface of the third lens is maintained. Therefore, a positive Petzval""s sum is lowered, the axial chromatic aberration and the lateral chromatic aberration can be preferably corrected in addition to the correction of the curvature of the image surface.
The conditional formula (8) indicates a condition for appropriately setting the positive refractive power of the air lens formed between the image side surface of the third lens and the object side surface of the fourth lens. The focal length fa of the air lens can be calculated according to a formula:
fa=R6xc2x7R7/{R7xc2x7(1xe2x88x92N3)+R6xc2x7(N4xe2x88x921)xe2x88x92D6xc2x7(1xe2x88x92N3)xc2x7(N4xe2x88x921)},
where N3 denotes the refractive index of the third lens at the d-line, N4 denotes a refractive index of the fourth lens at the d-line, R6 denotes a curvature radius of the image side surface of the third lens, R7 denotes a curvature radius of the object side surface of the fourth lens, and D6 denotes an interval of an air space on the optical axis between the third and fourth lenses.
When the conditional formula (8) is satisfied, the curvature of the image surface and the distortion can be corrected, and the telecentric characteristic of the light flux on the image side can be maintained.
Further preferably, in place of the conditional formula (8), a following conditional formula (9) is satisfied:
xe2x88x920.40 less than R5 /((N3xe2x88x921)xc2x7f) less than xe2x88x920.25xe2x80x83xe2x80x83(9).
Preferably, an image side surface of the fourth lens satisfies a following conditional formula (10):
Xxe2x88x92X0 less than 0xe2x80x83xe2x80x83(10)
for a displacement value X of an aspherical surface expressed in the formula (11):                     X        =                                                            h                2                            /              R8                                      1              +                                                1                  -                                                            (                                              1                        +                        K8                                            )                                        ⁢                                                                  h                        2                                            /                                              R8                        2                                                                                                                          +                      ∑                                          A                i                            ⁢                              h                i                                                                        (        11        )            
and a displacement value X0 of a rotational quadratic surface component of the aspherical surface expressed in the formula (12):                     X0        =                                            h              2                        /            R8                                1            +                                          1                -                                                      (                                          1                      +                      K8                                        )                                    ⁢                                                            h                      2                                        /                                          R8                      2                                                                                                                              (        12        )            
in a range of h satisfying hmax X 0.5 less than h less than hmax, where a vertex of the image side surface of the fourth lens is set as an origin, a direction of an optical axis is set as an X-axis, h denotes a height in an arbitrary direction perpendicular to the optical axis, Ai denotes an i-th order coefficient of the aspherical surface for the image side surface of the fourth lens, hmax denotes a maximum effective radius, R8 denotes a curvature radius of the image side surface of the fourth lens, and K8 denotes a conic constant for the image side surface of the fourth lens.
Here, the vertex of the image side surface denotes the intersection of the surface and the optical axis.
Generally, a plurality of lenses combined with each other are used. When the total lens length is shortened, a back focal length is shortened. Therefore, it is difficult to obtain the telecentric characteristic of the light flux on the image side. In the present invention, the image side surface of the fourth lens and nearest to the image side among the surfaces of the lenses is formed in the aspherical surface shape satisfying the formulas (10), (11) and (12). Therefore, the telecentric characteristic particularly for the light flux at the high angle of view can be maintained.
Preferably, the first lens is formed out of glass material, and the second, third and fourth lenses are formed out of plastic material.
The term xe2x80x9cformed out of plastic materialxe2x80x9d includes that the coating processing is performed on the surface of a substance including the plastic material as base material for the purpose of the anti-reflection or the improvement of surface hardness. This definition is available for following all description.
Plastic lenses made by the injection molding are used as the lenses composing the image pickup lens, the miniaturized lightweight image pickup lens is advantageously obtained at low cost. However, the refractive index of the plastic material considerably changes with the temperature. Therefore, assuming that plastic lenses are used for all lenses of the image pickup lens, a problem has arisen that the position of an image point of the whole image pickup lens changes with the temperature.
The configuration of the present invention uses many plastic lenses and compensates the change of the position of the image point of the whole image pickup lens caused by the temperature change. That is, the first positive lens is formed out of the glass material of which the refractive index hardly changes with the temperature, and the second, third and fourth lenses are formed out of the plastic material. The second lens has the comparatively strong positive refractive power, and the third lens has the comparatively strong negative refractive power. Therefore, the influence of the second lens on the position of the image point changing with the temperature cancels out that of the third lens, and the positional change of the image point in the whole image pickup lens caused by the temperature change can be suppressed to a low degree.
Preferably, a following conditional formula (13) is satisfied:
|f/f234| less than 0.7xe2x80x83xe2x80x83(13),
where f234 denotes a combined focal length of the second, third and fourth lenses, and f denotes a focal length of the whole image pickup lens.
The conditional formula (13) prescribes the combined focal length of the lenses formed out of the plastic material. Because the positional change of the image point of a plastic lens caused by the temperature change depends on a degree of the refractive power of the lens, the combined focal length of the first, second and third lenses formed out of the plastic material is set to a large value to suppress a sum of values of the refractive power of the lenses to a low value. Therefore, the positional change of the image point caused by the temperature change can be suppressed to a low degree.
Preferably, a saturated water absorption rate of the plastic material is not more than 0.7%.
Because the saturated water absorption rate of the plastic material is larger than that of the glass material, a non-uniform distribution of absorbed water occurs in the plastic lens due to a rapid change of humidity. Therefore, the refractive index of the plastic lens cannot be uniformly set, and a preferable image forming performance cannot be obtained. In the present invention, the lenses are formed out of the plastic material having a low saturated water absorption rate, and the deterioration of the performance caused by the change of humidity is reduced.
In accordance with the second aspect of the present invention, an image pickup lens comprising four lenses arranged in an order of a first lens, a second lens, a third lens and a fourth lens from an object side,
wherein the first lens has positive refractive power and has a convex surface facing toward the object side, one lens or two lenses selected from the second, third and fourth lenses have the positive refractive power, at least one lens of the positive refractive power selected from the first, second, third and fourth lenses is formed out of glass material,
another lens of the positive refractive power and one lens of negative refractive power selected from the first, second, third and fourth lenses are formed out of plastic material, and
an image side surface of the fourth lens satisfies a following conditional formula (14):
Xxe2x88x92X0 less than 0xe2x80x83xe2x80x83(14)
for a displacement value X of an aspherical surface expressed in the formula (15):                     X        =                                                            h                2                            /              R8                                      1              +                                                1                  -                                                            (                                              1                        +                        K8                                            )                                        ⁢                                                                  h                        2                                            /                                              R8                        2                                                                                                                          +                      ∑                                          A                i                            ⁢                              h                i                                                                        (        15        )            
and a displacement value X0 of a rotational quadratic surface component of the aspherical surface expressed in the formula (16):                     X0        =                                            h              2                        /            R8                                1            +                                          1                -                                                      (                                          1                      +                      K8                                        )                                    ⁢                                                            h                      2                                        /                                          R8                      2                                                                                                                              (        16        )            
in a range of h satisfying hmax X 0.5 less than h less than hmax, where a vertex of the image side surface of the fourth lens facing is set as an origin, a direction of an optical axis is set as an X-axis, h denotes a height in an arbitrary direction perpendicular to the optical axis, Ai denotes an i-th order coefficient of the aspherical surface for the image side surface of the fourth lens, hmax denotes a maximum effective radius, R8 denotes a curvature radius of the image side surface of the fourth lens, and K8 denotes a conic constant for the image side surface of the fourth lens.
Here, the vertex of the image side surface denotes the intersection of the surface and the optical axis.
In the above configuration, four lenses are arranged in an order of the first, second, third and fourth lenses. In use, the first lens is placed on the object side, and the fourth lens is placed on the image side. In this use condition, the first positive lens having the convex surface facing toward the image side and nearest to the object side is arranged. Therefore, as compared with the configuration having the first lens of a negative refractive power, the configuration advantageous to shorten the total lens length can be obtained.
Also, in the configuration of the present invention, lenses selected from the first, second, third and fourth lenses are formed out of the plastic material. As described above, though the plastic lens is advantageous to make a miniaturized lightweight image pickup lens at low cost, the plastic lens has a fault that the position of an image point of the lens considerably changes due to a large change of the refractive index of the lens caused by the temperature change.
Therefore, the configuration of the present invention uses many plastic lenses and compensates the change of the position of the image point of the whole image pickup lens caused by the temperature change. In the present invention, one positive lens is formed out of the glass material of which the refractive index hardly changes with the temperature, and two lenses including at least one positive lens and at least one negative lens are formed out of the plastic material. Therefore, the large positive refractive power can be distributed to the glass lens and the plastic lenses. Also, because the plastic lenses include the positive lens and the negative lens of the comparatively strong refractive power, the influence of the positive lens on the positional change of the image point caused by the temperature change cancels out that of the negative lens, and the positional change of the image point in the whole image pickup lens caused by the temperature change can be suppressed to a low degree.
Also, in the configuration of the present invention, the image side surface of the fourth lens is formed in the aspherical surface shape satisfying the formulas (14), (15) and (16). Therefore, the telecentric characteristic particularly for the light flux at the high angle of view can be maintained.
Preferably, the lenses other than the lens having the positive refractive power and formed out of the glass material are formed out of the plastic material.
In the above configuration, only one lens having the positive refractive power is formed out of the glass material, and the other three lenses are formed out of the plastic material. Therefore, many plastic lenses are used. Accordingly, the advantage of forming the lenses out of the plastic material can be effectively obtained.
Preferably, the first lens is formed out of the glass material.
In the above configuration, the first lens placed nearest to the object side and damaged most easily due to external causes is not formed out of the easily damaged plastic material but formed out of the glass material. Therefore, the generation of flaws of the first lens can be reduced, and the first lens protects the other lenses formed out of the plastic material.
Preferably, a following conditional formula (17) is satisfied:
|f/f234| less than 0.7xe2x80x83xe2x80x83(17),
where f234 denotes a combined focal length of the second, third and fourth lenses, and f denotes a focal length of the whole image pickup lens.
As described for the formula (9), the conditional formula (13) prescribes the combined focal length of the lenses formed out of the plastic material. Therefore, the combined focal length of the second, third and fourth lenses formed out of the plastic material is set to a high value according to the conditional formula (13) to suppress a sum of values of the refractive power of the lenses to a low value, and the positional change of the image point caused by the temperature change can be suppressed to a low degree.
Preferably, a saturated water absorption rate of the plastic material is not more than 0.7%.
Plastic having a low saturated water absorption rate is used as the lens material. Therefore, the performance of the plastic lenses does not deteriorate regardless of the change of humidity.
Preferably, following conditional formulas (18), (19) and (20) are satisfied:
L/2Y less than 1.60xe2x80x83xe2x80x83(18)
0.40 less than f12/f less than 0.70xe2x80x83xe2x80x83(19)
25 less than xcexdPxe2x88x92xcexdNxe2x80x83xe2x80x83(20),
where L denotes a distance on an optical axis from the object side surface of the first lens to an image side focal point of the whole image pickup lens, 2Y denotes the length of a diagonal line on an effective image screen, f12 denotes a combined focal length of the first lens and the second lens, f denotes a focal length of the whole image pickup lens, xcexdP denotes an Abbe number of the lens having the strongest positive refractive power, xcexdN denotes an Abbe number of the lens having the strongest negative refractive power.
The conditional formulas (18), (19) and (20) indicate conditions for obtaining a miniaturized image pickup lens in which the aberration is preferably corrected.
In the same manner as the formula (1), the conditional formula (18) indicates a condition for achieving the shortening of the total lens length.
Also, in the same manner as the formula (2), the conditional formula (19) indicates a condition for appropriately setting the combined focal length of the first lens and the second lens. When f12/f is set to be higher than the lower limit, the generation of both the high-order spherical aberration and the coma can be suppressed. Also, when f12/f is set to be lower than the upper limit, the total lens length can be shortened.
Also, the conditional formula (20) indicates a condition for prescribing the Abbe number of the positive lens having the strongest positive refractive power and the Abbe number of the negative lens having the strongest negative refractive power to preferably correct the chromatic aberration. When xcexdPxe2x88x92xcexdN is set to be higher than the lower limit, the axial chromatic aberration and the lateral chromatic aberration can be corrected in good balance.
Preferably, the second lens has the positive refractive power, and the third lens has the negative refractive power.
In the above configuration, because the positive lenses and the negative lens having the comparatively strong refractive power are arranged in that order from the object side, the total lens length can be easily shortened.
In accordance with the third aspect of the present invention, an image pickup unit comprises:
a solid-state image pickup element having a photoelectric transfer unit;
an image pickup lens for forming an image of an object in the photoelectric transfer unit of the solid-state image pickup element;
a substrate for holding the solid-state image pickup element, the substrate comprising an external connection terminal through which an electric signal is transmitted or received; and
a casing made of a light shielding substance and comprising an opening for incident light entering an object side,
wherein the solid-state image pickup element, the image pickup lens, the substrate and the casing are combined with each other,
a length of the image pickup unit in a direction of an optical axis of the image pickup lens is not more than 15 mm,
the image pickup lens comprises four lenses arranged in an order from the object side, and
the lens arranged nearest to the object side has the positive refractive power.
The xe2x80x9copening for incident lightxe2x80x9d is not limited to a space forming means such as a hole and indicates a means for forming an area through which incident light entering the object side can be transmitted. This definition is available for the fourth aspect of the present invention described later.
The image pickup unit having the configuration described above satisfies a request of the miniaturization and the lightening-in-weight and is used to be mounted on a small-sized portable electronic device. In this unit, the lens arranged nearest to the object side is set to a positive lens, and the structure advantageous for the shortening of the total lens length is adopted. The image pickup lens comprises four lenses while shortening the length of the whole image pickup unit in the direction of the optical axis.
The description xe2x80x9cthe length of the image pickup unit in the direction of the optical axis is not more than 15 mmxe2x80x9d denotes a total length of the image pickup unit having all the above-described elements along the direction of the optical axis. For example, when the casing is placed on a front surface of the substrate and electronic parts are arranged on a back surface of the substrate, a distance from an end portion of the casing on the object side to an end portion of the electronic parts projecting from the back surface of the substrate is not more than 15 mm. This definition is available for the fourth aspect of the present invention.
It is preferred that many lenses are arranged in the image pickup lens to correct various types aberration and improve the telecentric characteristic of the solid-state image pickup element and the quality of the formed image. However, the number of lenses of the image pickup unit mounted on a small-sized electronic device is generally three at most. Because four lenses are used for the image pickup unit of the present invention, the configuration of the present invention is advantageous for the improvement of the image quality.
Also, when the image pickup unit is miniaturized, the miniaturization of the lenses composing the image pickup lens is required, and the curvature radius of each lens necessarily becomes small. However, as the curvature radius is smaller, the difficulty in the keeping of the manufacturing precision of the lens is increased. In the present invention, because the image pickup lens comprises four lenses, the refractive power is distributed to the lenses, and a degree of the requirement for lowering the curvature radius of each lens can be reduced. Therefore, the manufacturing precision of the lens can be easily kept well, and the structure of the image pickup unit is advantageous for the improvement of the image quality.
Also, to lighten the whole image pickup unit, it is better to form each lens of the image pickup lens out of the plastic material in place of the glass material and to increase the number of plastic lenses.
However, the change of the refractive index of the plastic material caused by the temperature change is large. Therefore, when all lenses of the image pickup lens are formed of the plastic material, a problem has arisen that the position of the image point of the whole image pickup lens changes with the temperature. This positional change of the image point causes a problem particularly in an image pickup device (so-called pan-focus type image pickup device) on which a solid-state image pickup element having pixels densely arranged is mounted without having an auto-focus mechanism. Because a pixel pitch in the solid-state image pickup element having high dense pixels is small, a focal depth proportional to the pixel pitch becomes small, and an allowance width for the positional change of the image point becomes narrow. Also, the pan-focus type image pickup device is focused on an object spaced from the device by tens cm denoting a referential distance, and an image of an object spaced from the device by a distance ranging from an infinite distance to a very short distance near to zero is formed according to the depth of field. Therefore, the image of the object spaced by the infinite distance or the very short distance is slightly out of focus as compared with that spaced by the referential distance. When the position of the image point is changed due to the temperature change, a problem has arisen that the quality of the image of the object spaced by the infinite distance or the very short distance is extremely degraded.
To solve this problem, it is preferred that at least one lens is set to a glass lens and two lenses or more including at least one positive lens and at least one negative lens are set to plastic lenses. Thereby, a sum of values of the refractive power of the plastic lenses can be suppressed to a low value while maintaining the positive refractive power of the whole image pickup lens, and the influence of the positive lens on the positional change of the image point caused by the temperature change can cancel out that of the negative lens. To obtain the structure of the image pickup lens described above, when a degree of the design freedom is considered, the triplet lens structure is not adequate, and the image pickup lens having a quadruplet lens structure is required. Here, assuming that the image pickup lens has five lenses (quintuplet lens structure) or more, a large-sized image pickup unit is inevitably obtained. Therefore, the structure of five lenses or more is not adequate.
In accordance with the fourth aspect of the present invention, an image pickup unit comprises:
an image pickup lens having a photoelectric transfer unit;
one image pickup lens described above for forming an image of an object in the photoelectric transfer unit of the solid-state image pickup element;
a substrate for holding the solid-state image pickup element, the substrate comprising an external connection terminal through which an electric signal is transmitted or received; and
a casing made of a light shielding substance and comprising an opening for incident light entering an object side,
wherein the solid-state image pickup element, the image pickup lens, the substrate and the casing are combined with each other, and
a length of the image pickup unit in a direction of an optical axis of the image pickup lens is not more than 15 mm.
In the above configuration, each image pickup lens described above is mounted on the image pickup unit. Therefore, the image pickup unit having effects of the miniaturization, the lightening-in-weight and the high image quality can be actualized.
In accordance with the fifth aspect of the present invention, a portable terminal comprises the image pickup unit described above.
In the above configuration, the image pickup unit described above is mounted on the portable terminal. Therefore, the portable terminal having effects of the miniaturization, the lightening-in-weight and the high image quality can be actualized.